A gun may accelerate a projectile through a barrel by creating high pressure behind that projectile. Conventional guns employ some type of propellant to create that high pressure. The propellant may be gases resulting from burning gun powder, smokeless powder, rocket fuel, other chemical(s). Steam and other gasses have also been used. As a propellant pushes a projectile through a gun barrel, the propellant expands and the pressure in the propellant drops, thereby reducing the force accelerating the projectile. This pressure loss may be increased as heat energy in the propellant is transferred to the gun barrel.